The present invention relates to environmentally degradable polyolefin compositions and to environmentally degradable articles, films, and foams made from such compositions.
Thermoplastic synthetic resins are widely employed in the manufacture of diverse solid and foamed articles of commerce. Among the more well known plastic materials are polyethylene, polypropylene and polystyrene, all of which have been used in solid and foamed articles such as food packaging, films, bags, cups, cartons, trays and the like. Agriculture and construction trades are among the largest users of plastic materials, with applications such as agricultural mulching films, construction films, insulation, bags for fertilizers and pesticides, cartons and trays.
The huge volume of plastic products used by industry and consumers has raised concern among environmentalists about disposal. Environmentalists are concerned that the chemical inertness and stability of thermoplastic resins may contribute to solid waste disposal problems, because the conventional polymers do not degrade naturally within a reasonable period of time. During natural degradation, a total dissolution of an organic substance such as cellulosic products under the action of sun, heat, wind and rain, as well as fungi and microorganisms in the soil takes place.
However, unlike materials such as paper and cardboard, plastics are not readily destroyed by the elements of nature and are not easily degradable by microorganisms which attack most other forms of organic matter and return them to the biological life cycle. It has been estimated that it may take millions of years for organisms to evolve which are capable of performing this function.
The use of plastic materials in agriculture applications has a varying impact on the ecology and the economy. Produce is packaged in plastic crates, tote boxes and trays. Fertilizers and chemicals are brought to the farm in plastic packaging materials such as shipping bags or plastic containers. Plastic bailing twine and plastic irrigation tubing are used. A very important application for plastic is in agricultural films. These films make it possible to grow crops in areas where the climate is a limiting factor. In locations where the same crops have been grown traditionally, a marked increase in the quantity and quality of crops grown with the use of agricultural films can be obtained. Moreover, in many cases a reduction in fertilizers, pesticides and herbicides is made possible which represents not only a great savings in cost, but also reduces the damage caused by these chemicals to the environment. However, recovery of the films, which are spread over large areas, presents an arduous and costly task. Recovery must be effected, lest further cultivation of the land becomes practically impossible.
Various approaches to produce environmentally degradable polymer compositions for use in the fabrication of containers, films, bags and the like have been essayed. One approach provides for the incorporation of photosensitive additives to a host polymer to impart environmental degradability to the articles, particularly by photodegradation mechanisms. The use of such additive systems has caused substantial problems in the past. For example, the polymer photosensitive additive systems have exhibited a spontaneous separation of the components or component migration to the surface of the article during and after melt extrusion. The additive systems are often subject to discoloration and odor generation during processing of the melt, requiring the addition of dye colorants to maintain an acceptable product appearance. Such systems have had poor extrusion viscosity characteristics and poor thermoformability and have not exhibited the requisite stiffness or flexibility for certain applications.
Another approach involves the blending of high density polyethylene or medium density polyethylene with an ethylene-carbon monoxide copolymer, such as disclosed in German patent document No. 2316697 entitled "Polymeric Substance Photo-decomposable by the Action of Ultraviolet Irradiation". However, such blends degrade too rapidly for successful use in articles requiring flexibility.
Cerium octoate has been said to enhance the degradability of ethylene polymers in the presence of ultraviolet light or ionizing radiation, that is, when the polyethylene composition is irradiated after use and prior to disposal. However, such radiation is not always present, and it would be desirable to have polyolefins that are degradable under other circumstances as well.
There continues to exist a need for environmentally degradable polymeric compositions which have sufficient mechanical properties for end use applications such as food packaging, agriculture and construction films and foams such as food wrapping, agricultural mulch film, beverage loop carriers and grocery sacks, and which exhibit balanced environmental degradation, excellent mechanical properties, extrudability, thermoformability, and do not spontaneously suffer component migration or separation upon melt extrusion formation or during use.